A negative glow discharge lamp typically is comprised of a light-transmitting envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope which is adapted to emit visible light upon absorption of ultraviolet radiation that occurs when the lamp is excited. The lamp is excited by means of the application of a voltage between the lamp electrodes. Current flows between the electrodes after a certain potential is applied to the electrodes, commonly referred to as the breakdown voltage. An elementary explanation of the phenomenon is that the gas between the electrodes becomes ionized at a certain voltage, conducts current, and emits ultraviolet radiation. The phosphor coating on the inner surface of the lamp envelope is caused to fluoresce and re-emit a substantial portion of the ultraviolet radiation as visible light. The spectral characteristics of the visible light is determined by the composition of the fluorescent powders used for the phosphor coating.
Negative glow discharge lamps, in common with other discharge devices, generally require a series-connected current-limiting device. The reason for this requirement is commonly given as the "negative resistance" of the discharge, a statement that implies that the discharge has its differential resistance much smaller than its static resistance (i.e., dV/dI &lt;&lt;V/I).
Examples of typical glow discharge lamps are found in U.S. Pat. No. 2,341,990 to Inman et al and U.S. Pat. No. 2,403,184 to Lemmers.
Reference is now made herein to FIG. 1 which illustrates a known circuit diagram for starting and operating a typical negative glow discharge lamp. FIG. 1 illustrates a glow discharge lamp 10' including an anode A and a cathode C. The cathode C is in the form of a standard filamentary exciter coil coated with an emissive material. Continuous heater current is provided to cathode C by means of a first dc source 13. A second dc source 15 is provided and is electrically coupled to lamp 10' through a series connected external ballast, such as a resistor R.sub.b.
During lamp operation, a considerable voltage drop exists on the cathode due to electrical heating. As a result, the discharge current (indicated by arrows on lamp 10') flows to the negative end of the cathode and thereby bypassing the resistance of the cathode. While the filament power goes for heating the entire cathode surface, only a small portion of the cathode surface takes part in actual current conducting . The concentration of the discharge current on this small portion of the cathode surface leads to excessive heating and evaporation of the oxide coating of the cathode causing the formation of a hot spot. This is detrimental to the maintenance (i.e., light output versus time) and results in phosphor darkening.